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And in a letter to MPs, Theresa May yesterday signalled that other blunders included an immigration officer at passport control missed an alert and officials overseas missed it because alerts are not monitored around the clock.

The Home Secretary also revealed she is looking at creating a new criminal offence of knowingly entering the UK when you are banned.

In a letter to the Commons Home Affairs Select Committee, Mrs May said an investigation in to the incident found "insufficiently robust processes" led to six missed chances to intervene overseas, on departure to, and in the UK.

A new system will be brought in to highlight live alerts to UK ports and to the agency's overseas network, while the police national computer will be amended to include details of those banned from the country, she said.

An investigation by Her Majesty's Inspectorate of Constabulary (HMIC) has made eight recommendations to improve the system and the UK Border Agency has been asked to comply with them "as quickly as possible”.

Mrs May said: "I was very concerned that an individual who was excluded for unacceptable behaviour was able to enter the UK and I was very grateful to the Chief Inspector of Constabulary for undertaking an urgent review into the circumstances of the case.

"The HMIC investigation specifically pointed to insufficiently robust processes being in place leading to six missed opportunities for intervention (overseas, on departure to and in the UK) and made eight recommendations.

"I have accepted all of these recommendations and have instructed UKBA to comply with them as quickly as possible."

The investigation, by Chief Inspector of Constabulary Sir Denis O'Connor, called for a thorough review of the UKBA's system for issuing and responding to alerts, saying they should be "monitored by overseas staff seven days a week".

A prioritisation process should be developed and the passports scanning process should be reviewed to ensure border officials cannot overlook an alert, he said.

He also found that the UKBA should have a "heightened response to excluded persons arriving in the UK and ensure that such individuals are intercepted at the earliest opportunity".

Mr Salah, 52, the leader of the Islamic Movement in Israel, has allegedly spread a conspiracy theory that Jewish people were involved in the September 11 attacks.

A High Court judge is currently deciding whether Mr Salah should be entitled to damages.

He was held in detention for three weeks as officials sought to deport him before being freed on bail. He remains in the UK.